mpc8260/i2c.c: replace PRINTD() by debug()

This also fixes some GCC 4.6 build warnings like:
i2c.c: In function 'i2c_init':
i2c.c:221:26: warning: variable 'txbd' set but not used
[-Wunused-but-set-variable]
i2c.c:221:19: warning: variable 'rxbd' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Heiko Schocher <hs@denx.de>